In recent years, the use of memory devices having nonvolatile memories, such as USB memories, flash memory cards and flash disks, has become widespread. The following multiple-value technique is known to increase the capacity of such a memory device by enabling three- or more-valued data to be stored in one memory cell: the number of voltage thresholds of each memory cell is increased as illustrated in FIG. 28, and, in the case of a four-valued memory cell, each of 2-bit data items, “11,” “10,” “01” and “00,” is linked to a corresponding voltage level.
As a related conventional technique, a nonvolatile semiconductor storage device and a data storage system are known for having a large capacity and ensuring high levels of reliability and high-speed operations (see Patent Document 1, for example).    [Patent Document 1] Japanese Laid-open Patent Publication No. 2001-210082